1. Field of the Invention
This invention relates to non-invasive means of determining cardiac output in patients, and specifically relates to partial re-breathing systems and methods for determining cardiac output in patients.
2. Statement of the Art
It is important in many medical procedures to determine or monitor the cardiac output of a patient. Techniques are known and used in the art which employ the use of catheters inserted at certain arterial points (e.g., femoral artery, jugular vein, etc.) to monitor blood temperature and pressure in order to determine cardiac output of the patient. Although such techniques can produce a reasonably accurate result, the invasive nature of the procedure has high potential for morbidity and mortality consequences.
Adolph Fick's measurement of cardiac output, first proposed in 1870, has served as the standard by which all other means of determining cardiac output have been evaluated since that date. Fick's well-known equation, written for CO.sub.2, is: ##EQU1##
where Q is cardiac output, V.sub.CO.sub..sub.2 is the amount of CO.sub.2 excreted by the lungs and ##EQU2##
are the arterial and venous CO.sub.2 concentrations, respectively. Notably, the Fick Equation presumes an invasive method (i.e., catheterization) of calculating cardiac output because the arterial and mixed venous blood must be sampled in order to determine arterial and venous CO.sub.2 concentrations.
It has previously been shown, however, that non-invasive means may be used for determining cardiac output while still using principles embodied in the Fick Equation. That is, expired CO.sub.2 ("pCO.sub.2 ") levels can be monitored to estimate arterial CO.sub.2 concentrations and a varied form of the Fick Equation can be applied to evaluate observed changes in pCO.sub.2 to estimate cardiac output. One use of the Fick Equation to determine cardiac output in non-invasive procedures requires the comparison of a "standard" ventilation event to a sudden change in ventilation which causes a change in expired CO.sub.2 values and a change in excreted volume of CO.sub.2. The commonly practiced means of providing a sudden change in effective ventilation is to cause the ventilated patient to re-breath a specified amount of previously exhaled air. This technique has commonly been called "re-breathing."
Prior methods of re-breathing have used the partial pressure of end-tidal CO.sub.2 to approximate arterial CO.sub.2 while the lungs act as a tonometer to measure venous CO.sub.2. That method of re-breathing has not proven to be a satisfactory means of measuring cardiac output because the patient is required to breath directly into and from a closed volume in order to produce the necessary effect. However, it is usually impossible for sedated or unconscious patients to actively participate in inhaling and exhaling into a bag. The work of some researchers demonstrated that the Fick Equation could be further modified to eliminate the need to directly calculate venous P.sub.CO.sub..sub.2 (P.sub.VCO.sub..sub.2 ) by assuming that the P.sub.VCO.sub..sub.2 does not change within the time period of the perturbation, an assumption that could be made by employing the partial re-breathing method. (See, Capek et al., "Noninvasive Measurement of Cardiac Output Using Partial CO.sub.2 Rebreathing", IEEE Transactions On Biomedical Engineering, Vol. 35, No. 9, September1988, pp. 653-661.)
Known partial re-breathing methods are advantageous over invasive measuring techniques because they 1) are non-invasive, 2) use the accepted Fick principle of calculation, 3) are easily automated, 4) require no patient cooperation and 5) allow cardiac output to be calculated from commonly monitored clinical signals. However, known partial re-breathing methods have significant disadvantages as well. Specifically, known methods 1) are less accurate with non-intubated or spontaneously breathing patients, 2) only allow intermittent measurements (usually about every four minutes), 3) result in an observed slight, but generally clinically insignificant, increase in arterial CO.sub.2 levels and 4) do not permit measurement of shunted blood flow (that is, blood which does not participate in gas exchange). Further, known apparatus used for partial re-breathing techniques are of standard construction and do not compensate for differences in patient size or capacities. In addition, many devices employ expensive elements such as three-way valves, which render the devices too expensive to be used as disposable units.
Thus, it would be advantageous to provide a means of measuring cardiac output using partial re-breathing techniques which 1) overcome the disadvantages of prior systems, 2) provide better and more continuous measurement, and 3) require less expensive equipment, thereby making the device suitable for manufacturing as a single-use, or disposable, product. It would also be advantageous to provide partial re-breathing apparatus which is instantaneously adjustable to compensate for various sizes and capacities of patients. Further, it would be advantageous to provide new methods of estimating cardiac output based on alveolar CO.sub.2 output rather than end-tidal CO.sub.2 as is currently used in the art.